Synthesis Of High Softening-Point Polycabosilane And Preparation Of Low Oxygen Content Sic Fibers

The chemical vapour curing?CVC?method has advantages of low production cost and easy implementation to prepare SiC fibers with low oxygen content.The current spinning-level polycarbosilane?PCS?can not be cured via CVC method owing to its low softening point and molecular weight.In the present paper,we report a novel strategy to prepare PCS with high softening point and good spinnability by modifying the conventional PCS.Finally,SiC fibers with low oxygen content have been success fully prepared by using these PCS precursors of high softening point.The high-softening-point PCS was synthesized by increasing the reaction temperature and time using normal-pressure high-temperature method.Under the reaction conditions,the softening point and molecular weight of PCS can grow up in two different ways,including the steady growth pattern and the accelerated growth pattern.In the steady growth pattern,the main reaction was the condensation polymerization of low-molecular-weight PCS to form the middle-molecular-weight PCS.The PCS obtained by steady growth pattern had a bimodal molecular weight distribution according to the GPC curves and possessed good spinnability.In the accelerated growth pattern,high-molecular-weight fraction of PCS grew rapidly,thus molecules of ultra-high and even extremely high molecular weight were obtained.The PCS formed through the accelerated growth model contained many branching structures,which exhibited multi-peak molecular weight distribution and poor spinnability.PCS?PCS-10?with high softening point and good spinnability was successful synthesized by controlling the condensation polymerization condition to make it progress in the steady growth model.The softening point of this PCS was 256²87?,with the number average molecular weight as 2275,and the molecular formula as SiC1.82H6.64O0.02.However,the spinnability of PCS-10 was not good enough for preparing continuous SiC fibers.It could be attributed to the poor-control of the intermolecular condensation reaction by normal-pressure high-temperature method and the formation of branched structures in PCS.The high-softening-point PCS was fabricated through an improved bridging method by the reaction of the low-softening-point polycarbosilane?LPCS?with tetramethyldivinyldisilazane?TMDS?or divinyldimethylsilane?DVS?.It was found that the hydrosilylation reaction between Si-H groups of LPCS and vinyl groups of the bridging reagent carried out in a stepwise manner.The LPCS reacted with one of two vinyl bonds of the bridging reagent to form a hanging structure in the initial stage of the reaction.Subsequently,another LPCS molecule reacted with the hanging structure to form a bridged structure until the hydrosilylation reaction was completed at 350?.The molecular weight and softening point of the PCS increased due to the hydrosilylation reaction.The softening point,molecular weight and spinnability of PCS can be adjusted by controlling the composition and structure of PCS through choosing LPCS of suitable softening point and changing mass ratio of the bridging reagent to LPCS.By using the TMDS as the bridging reagent,the high-softening-point PCS?TPCS-3?with a softening point of 244²78?,a number average molecular weight of 2544g/mol,a molecular formula of SiC1.94H6.17O0.02N0.03 and good spinnability was obtained by optimizing the mass ratio of TMDS to LPCS as 0.08 in the reaction carried at 350? for 2h under atmospheric pressure.However,the spinnability of TPCS became more poor above 400? due to the reaction between Si-H and N-H groups.By using the DVS as the bridging reagent,and LPCS?Ts=185¹97??as raw material,with a pre-charged pressure of 5MPa,the high-softening-point PCS?DPCS-4?with a softening point of 262? was synthesized by controlling the mass ratio of DVS to LPCS as 0.04.The number average molecular weight of the PCS was 2454g/mol,which had a molecular formula of SiC1.91H7.08O0.03 and good spinnability.The spinnability of DPCS-4 was as good as that of KD-PCS.By comparison with normal-pressure high-temperature method,the bridging method showed advantages of good control and high efficiency of the intermolecular reaction with a simple process.By optimizing the melt-spinning conditions of DPCS,the DPCS fibers with the diameter of 12¹4mm were prepared by stable continuous spinning.After treating the DPCS-4 fibers in cyclohexene atmosphere at 150³50? by CVC method,the cured DPCS-4?CVC-PCS?fibers were obtained.The oxygen content of CVC-PCS fibers was lower than 1.0wt%.After sintering at 1400?,the CVC-SiC fibers with oxygen content of 3.24wt% were prepared.The tensile strength of CVC-SiC fibers was 2.56 GPa with the elastic modulus of 225 GPa.The chemical formula of CVC-SiC fibers was confirmed as SiC₁.45O₀.10.The oxygen content of CVC-SiC fibers was close to that of EB-SiC fibers?SiC₁.43O₀.07?which were cured by electron beam radiation.There was a very thin carbon rich layer??4 nm?and an intermediate layer??25 nm?on the surface of CVC-SiC fibers.The carbon content decreased within the range of 30 nm from the surface towards the interior and became stable in the core of the fiber.The fibers were composed of a large number of ?-SiC mirco-grains with a size of 3.5 nm,which was also close to that of EB-SiC?4.1nm?.Simultaneously,graphited free carbon was observed in the fiber as well.The CVC-SiC fibers exhibited excellent mechanical strength,thermal stability and oxidation resistance.After heating at 1600? for 1h in Ar atmosphere,the tensile strength of CVC-SiC fibers was 1.46 GPa,which was 57% of the initial strength.After oxidizing at 1300? in air for 1h,the tensile strength was 1.27 GPa,which maintained 50% of the initial strength.The CVC-SiC fibers showed comparable high-temperature and oxidation resistance by comparing with EB-SiC fibers.